1. Field of the Invention
The invention relates to an air electrode for a metal-air battery, which functions stably both upon oxygen reduction during discharging and upon oxygen generation during charging, a membrane/air electrode assembly that has the air electrode, and to a metal-air battery.
2. Description of Related Art
A metal-air battery is a chargeable/dischargeable battery that utilizes a metal or a metal compound in a negative electrode active material and oxygen in a positive electrode active material. Oxygen, as the positive electrode active material, can be obtained from air, and hence the positive electrode active material need not be sealed in the battery. Therefore, metal-air battery realizes, in theory, a capacity that is greater than that of secondary battery that utilizes solid positive electrode active materials.
In a case where an alkaline electrolyte solution is used, the reaction represented by formula (I) below takes place at the air electrode of a metal-air secondary battery during discharge.O2+2H2O+4e−→4OH−  (I)
The OH− ions generated in the reaction of formula (I) are dissolved in the electrolyte solution.
The reaction represented by formula (II) takes place at the air electrode during charging.4OH−→O2+2H2O+4e−  (II)
The O2 gas generated in the reaction of formula (II) diffuses out of a cell.
The reaction of formula (I) is believed to take place at so-called three-phase boundary at which three phases come into contact, namely oxygen as a reactant, the air electrode catalyst, and an ion-conducting phase. Accordingly, three-phase boundary may be formed in significant numbers in order to promote the reaction.
Recent years an active ongoing research relating to metal-air secondary battery that employs alkaline electrolyte solutions has taken place. If an alkaline electrolyte solution is used, the latter absorbs readily carbon dioxide present in air. Absorption of carbon dioxide impairs the performance of the electrolyte solution and causes precipitation of carbonate salts of metal ions, all of which is problematic. In order to avert the above problem, Naoko FUJIWARA “Kinzoku-kuki nijinidenchiyo kagyaku kukikyoku no kaihatsu” (Development of Reversible Air Electrode for Metal-Air Secondary Battery (Electrochemistry, 78 (2010) 540-544)) proposes an electrode structure where a layer of an anion-exchange membrane is provided between an air electrode and an alkaline electrolyte solution. The anion-exchange membrane is a polymer membrane that selectively lets anions through, such that the membrane suppresses migration of metal ions in the electrolyte solution towards the air electrode, and dissolution of carbon dioxide from air into the electrolyte solution, without hampering the reactions of formulas (I) and (II).
Naoko FUJIWARA “Kinzoku-kuki nijinidenchiyo kagyaku kukikyoku no kaihatsu” (Development of Reversible Air Electrode for Metal-Air Secondary Battery (Electrochemistry, 78 (2010) 540-544)) studies the feature of nullifying the influence of carbon dioxide through contact with an anion-exchange membrane in an air electrode that uses a noble metal catalyst. In such an electrode structure, however, the air electrode is not in direct contact with the electrolyte solution, and hence three-phase boundaries do not form readily in a catalyst layer.